This invention relates generally to storage tanks having roofs that float on the surface of the stored product, and more particularly to secondary seals used in such tanks.
Floating roof tanks are widely used to store volatile petroleum-based liquids and limit the quantity of product evaporative emissions that may escape to the environment. Such tanks may be configured either as internal floating-roof tanks or as external floating-roof tanks. In each configuration, the floating roof is designed to remain in contact with the product liquid surface and cover all of the surface of the product except for a small annular surface area between the outermost rim of the floating roof and the inside surface of the tank shell. Product evaporative emissions from this area may be controlled by a single, primary seal. However, for increased effectiveness, emissions from this area are conventionally controlled by a combination of perimeter rim seals, including a primary seal with a secondary seal mounted in the rim space above it.
Primary seals conventionally take the form of a piece of fabric extending between the floating roof and a shoe plate that bears on the tank shell. Examples of such seals are illustrated in Wagoner, U.S. Pat. No. 5,036,995 and in Ford et al., U.S. Pat. No. 5,529,200. Alternatively, primary seals may be in the form of resilient liquid- or foam-filled seals that are supported from the floating roof.
Secondary seals for floating-roof tanks should span the distance between the floating-roof and the tank shell. Most conventional secondary seals are mounted to the floating roof and extend upwards across the rim space to contact the tank shell some vertical distance above the floating roof. The vertical distance represents a characteristic clearance requirement for the secondary seal.
One prevalent type of secondary seal includes metal compression plates that attach to the floating roof and support a tip seal against the tank shell, as disclosed in Kinghorn et al., U.S. Pat. No. 4,116,358; Grove et al., U.S. Pat. No. 4,615,458; and Thiltgen et al., U.S. Pat. No. 4,308,968. In each of these designs, the compression plates are mounted at an angle to the tank shell.
The angle of the compression plates is critical. If the angle is too steep, the tip seal can become jammed against the tank shell as the seal attempts to pass over weld seams or other surface irregularities on the tank shell. If the angle is too shallow, the tip seal can drag against the tank shell or catch on a weld seam or other shell discontinuity. Either event may cause the compression plates to fold into the rim space and damage one or more sections of the secondary seal, opening gaps between the tip seal and the tank shell that can lead to increased evaporative emissions to the atmosphere.
Further, as a floating roof drifts toward one section of the tank shell, the angle of the compression plates becomes more vertical, increasing the vertical clearance required to keep the tip seal inside the tank and in contact with the tank shell. For a typical storage tank with a nominal 8xe2x80x3 rim space, the width of the rim space at any particular point may actually vary between about 4xe2x80x3 to more than 12xe2x80x3 as the roof moves, increasing the vertical clearance requirement to as much as 24xe2x80x3. Tank size or tank foundation considerations may also dictate a 10-inch or even 12-inch nominal width for the rim space, with permissible variations as large as xc2x17 inches or more. Consequently, the vertical clearance requirement for a conventional secondary seals may sometimes exceed 31xe2x80x3.
This vertical clearance requirement presents a problem both for new tanks and for retrofitting old tanks. New tanks must be designed with excess, unusable capacity to account for the required vertical clearance, adding to the construction cost. Similarly, when a secondary seal is added to an existing floating-roof tank, the maximum filling height of the tank may need to be reduced to accommodate the required vertical clearance for the secondary seal. Any such reduction of the maximum filling height represents lost inventory to the owner/operator of the tank. For example, when a secondary seal is added to an existing 100-foot (≈30 meter) diameter floating-roof tank, a nominal 2-foot (0.6 meter) reduction in filling height represents a loss of approximately 117,500 (2800 Bbl) of product storage. Such a loss can significantly reduce the revenue of the owner/operator of the tank.
It is believed that previous efforts to solve the problems associated with the vertical clearance requirement have not found commercial success. Hills et al., U.S. Pat. No. 104,339,052, discloses a secondary seal in the form of a tube that is connected near the top of the floating roof. One problem with this arrangement is that the secondary seal can rotate upwards, out of the rim space as the floating roof descends during product send-out operations. Petri et al., U.S. Pat. No. 5,284,269, discloses a space-saving double-seal system comprised of two shoe segments mounted above each other. One problem with this arrangement is that the shoe supports of the primary seal extend beneath the floating roof, increasing the risk of interference with equipment inside the tank.
Because of these disadvantages in previously-disclosed low-profile secondary seals, it is believed that a need exists for a novel, low-profile secondary seal.
The present invention provides a useful, low-profile secondary seal that can be used with a conventional primary seal that utilizes a shoe plate. The secondary seal is positioned above the primary seal and comprises a resilient tube connected to the floating roof. A tip seal adjacent the shell of the tank and is connected to the shoe plate by a spacer. In use, the tube bears on the spacer with sufficient force to maintain the tip seal in sealing engagement against the shell of the tank.
In some embodiments of the invention, the tip seal may be no more than about twelve inches above the top of the floating roof. The spacer may be constructed in the form of a series of inwardly-projecting, overlapping plates. The tube may be attached to the shoes by a flap that extends from the tubular section. A protective cover may also be inserted between the shoe plate and the tube. Electrical shunts may extend from the tip seal to the floating roof, and from the tip seal to the shell.